The invention relates to a method for the determination of the thickness of transparent, clear or colored, layers of lacquer on bright rolled, metallic foils or thin strips, in particular of aluminum or aluminum alloys, wherein the optical transmission factor of the layer is determined. The invention further relates to apparatus for carrying our the method.
The industrial manufacturers of foils or thin strips rely on the fact that the lacquer invariably needs to be applied in a certain thickness. An objective color comparison is necessary, in particular in the case of the application of transparent coloured lacquers. By means of so-called colorimetrics, the colors are expressed in numerical terms.
It is known that the thickness of transparent layers can be measured in that the absorption of infrared or visible light is measured in a narrow spectral range. For this purpose the Lambert-Beer law known from classical physics is applied: EQU I.sub.D =I.sub.o .multidot.e.sup.-k.c.s
In this expression, I.sub.D represents the luminous energy emerging from the transparent layer, I.sub.o represents the luminous energy irradiated, k represents the molar extinction coefficient, c represents the concentration of the transparent dye and s represents the thickness of the layer of the transparent homogeneous medium.
The natural logarithm of the proportion I.sub.D /I.sub.o of the luminous energy which is transmitted by the transparent layer, i.e. of the transmission factor D, thus amounts to -k.c.s.
In the case of the application of a homogeneous transparent layer, a change in the transmission factor permits a direct conclusion to be drawn regarding a change in the layer thickness, because not only the extinction coefficient k but also the concentration c remains constant.
In the existing practice of the coating of foils or thin strips, either the application of lacquer is monitored gravimetrically (in the case of clear lacquers) or the tone of the hue is monitored visually or by means of a colorimeter (in the case of transparent colored lacquers) using samples from the beginning and end of a roll. Any possible changes in the application of the lacquer, and thus possibly in the tone of the hue, which occur during the coating of a roll of foil, which has a running length of some thousands of meters, can thus only be detected at the end. This leads potentially to considerable losses of material and labour.
The inventors set themselves the object of providing a method and apparatus by which, in the case of moving foils or thin strips, in particular of aluminum or aluminum alloys, the thickness or the tone of the hue of the applied transparent, clear or colored, layers of lacquer can be measured continuously, on a non-contact basis and with great accuracy, it being possible for any changes to be indicated or automatically corrected directly after occurrence thereof.